1. Field of the Invention
The present invention relates to ultra wideband pulse modulation and detection techniques, and more specifically, it relates to techniques for overcoming multiple access interference in ultra wideband transmissions.
2. Description of Related Art
Multiple access interference (MAI) is a serious problem in UWB communication systems. The use of multiple mutually orthogonal pulses in a multiuser UWB channel can minimize performance degradation due to MAI. Conventionally the multiuser TR receivers use different delays for each user. The one problem with this method is that the same pulse shape for all users generates strong MAI, another major challenge in this technique is delay element implementation for each user which is difficult to incorporate in low power integrated systems.
An example of the use of delay in multiple orthogonal transmitted-reference pulses for UWB channelization is described in U.S. Pat. No. 7,194,019, titled: “Multi-pulse multi-delay (MPMD) multiple access modulation for UWB”, incorporated herein by reference. The modulation technique provided in the patent utilizes multiple orthogonal transmitted-reference pulses for UWB channelization. The UWB receiver samples the second order statistical function at both zero and non-zero lags and matches the samples to stored second order statistical functions, thus sampling and matching the shape of second order statistical functions rather than just the shape of the received pulses. While presenting systems and methods that are capable of delivering a large amount of data with a low power spectral density by sampling the ACF of received signals and matching it to the ACF samples of the original pulses for each user and further presenting systems that are more robust and have increased channel capacity to allow a greater number of users to communicate, the required delay for each user presents a significant implementation challenge.
A prior art modulation scheme is known as UWB Slightly Frequency Shifted Transmit Reference (UWB SFS-TR). An example of this technique is described by Qu Zhang in a paper titled: “Slightly Frequency-Shifted Reference Ultra-Wideband Radio”, incorporated herein by reference. UWB SFS-TR offers the advantages of the conventional TR technique, including simplicity, reliability of communications in multipath channels, and automatic channel estimation, and avoidance of the complications related to delay element implementation in standard TR receivers. However, UWB SFS-TR does not address the multiuser detection challenge.